1. Field of the Invention
The present invention relates to an energy absorption system that incorporates an internally liquid-cooled disc and, more particularly, to an improved energy absorption system wherein the internally liquid-cooled disc is in the form of a one piece casting.
2. Description of the Prior Art
Disc braking systems have been used for many years to brake automobiles, aircraft, trucks, and other vehicles. Such braking systems are also used as dynamometers to absorb kinetic energy associated with systems that test power output from power plants, engines, and the like. Disc braking systems are chosen over other braking means, such as drum brakes, for various reasons including braking effectiveness (anti-fading), cost, and serviceability.
Generally, a disc braking system includes a rotating disc upon which a braking or retarding force is applied. The rotating disc is connected to a rotating shaft and stationary brake pads are forced against the rotating disc to effectuate the braking action. The stationary brake pads are held in place by calipers that are normally hydraulically controlled; that is, hydraulic pressure forces the brake pads against the row, ting disc. The rotating disc possesses a kinetic energy which is transformed into heat via friction as the disc decelerates between the brake pads. Under normal conditions, such as decelerating an automobile operating at normal highway speed, the heat generated by the friction is dissipated from the disc and the brake pads to the surrounding air. However, during longer and harder periods of braking, such as braking a car or truck while descending long downhill grades, the disc braking system may no longer be cooled at an adequate rate, thereby adversely affecting their braking capability. When such operating conditions are encountered, the disc and brake pads become excessively hot and deteriorate rapidly.
The deterioration of the disc braking system becomes especially apparent for conditions where there is a high energy transfer rate, such as a kinetic energy transfer associated with rapid braking of a racing car traveling at high speed. In such situations, the use of air to cool the disc braking system is generally insufficient to prevent excessive disc and brake pad wear. Other more effective heat transfer methods have been utilized. One such method involves spraying a liquid, such as water, directly on the rotating disc as it is braking, thus allowing the heat to be transferred to the liquid. This method, though increasing the heat transfer rate, creates a braking hazard because the coefficient of friction between the disc and the pads will vary dramatically as a function of disc/pads temperature and the amount of liquid between the disc and the pads. Thus, an externally liquid-cooled disc braking system, though extending the disc and brake pads lives, can create unreasonable risks and cannot be effectively controlled.
Dynamometers are devices for absorbing and measuring energy output of power plants, engines, or other mechanical energy producing devices (MEPD's). In operation, the MEPD can be directly connected, via a crankshaft, or indirectly coupled, via a power transfer means (i.e. a vehicle's drive wheel), to the dynamometer. A retarding torque is applied to the MEPD by the dynamometer. The MEPD's output torque is then measured at a resultant angular speed. The dynamometer must be capable of applying sufficient reactive torque while effectively dissipating the absorbed energy through heat transfer so as to maintain the dynamometer within a safe operating range.
Various methods of power absorption have been utilized in dynamometers. A dynamometer employing a disc braking system using air cooling has restricted power absorption capability due to heat transfer limits of the air contacting the disc and pads. Such dynamometers are therefore restricted to testing MEPD's of low power capability, with such testing generally limited to short periods of time. Also, externally liquid-cooled disc brakes which spray liquid onto the disc to dissipate heat are not particularly suitable for dynamometers since such cooling is generally not uniform, therefore making accurate torque measurement difficult.
Some internally liquid-cooled disc braking systems have been developed to overcome the difficulties associated with externally liquid-cooled disc braking systems. The internally liquid-cooled disc braking systems operate by injecting a cooling liquid into the disc, which has an internal cavity. The heat generated during braking is transferred to the liquid, and the liquid, now at a higher temperature, is forced out of the disc. The system can be either open or closed loop and in either case the cooling is generally uniform and the liquid does not come between the disc and the brake pads. Thus, an internally liquid-cooled disc braking system overcomes the difficulties associated with externally liquid-cooled disc braking systems.
In some instances, the amount of energy absorbed by an internally liquid-cooled disc braking system is large enough to vaporize the liquid. Such instances typically result in maximum heat transfer for a given change in temperature, a condition commonly known as the latent heat of vaporization of the liquid. Thus, internally liquid-cooled braking systems which operate at the latent heat of vaporization of the liquid typically provide maximum energy absorption.
Although maximum heat transfer occurs at vaporization, other conditions result which can cause problems for internally liquid-cooled systems. More specifically, the liquid vaporization, if not allowed to expand, generates a large amount of pressure which can block the entry of incoming liquid. If the cooling liquid is blocked, a condition known as vapor lock, the vaporized liquid will superheat, thereby disabling the energy absorbing effect.
Various designs for internally liquid-cooled braking systems are disclosed in several prior art devices. In particular, U.S. Pat. No. 5,003,829, entitled, Energy Absorbing Device and Torque Measuring Apparatus Therefor, assigned to the inventor of the present invention, and hereby incorporated by reference, discloses an internally liquid-cooled disc braking system that disperses cooling liquid to cooling cells formed internal to the disc and utilizes the latent heat of vaporization of the liquid for maximum cooling. The problem of vapor lock is overcome by directing the cooling liquid into the cooling cells, through flow tubes positioned within the cooling cells, before vaporization can occur. A centripetal force on the liquid through the flow tubes then forces the liquid/vapor out of the cells. In this system 10, shown in FIG. 1, the cooling cells 12 are molded into two disc halves 14a and 14b, which are welded together after the flow tubes 16 are inserted in place. The welding must be performed such that the cooling cells 12 on the two disc halves 14a and 14b are aligned and the cooling liquid within the cells 12 does not leak. Such a welding process can be time consuming and the weld itself could crack as a result of the thermal variations inherent in the system 10. The present invention alleviates these concerns with the fabrication of an internally liquid-cooled disc in the form of a one piece casting.
Other designs for internally liquid-cooled braking systems are disclosed in U.S. Pat. Nos. 2,982,377, 2,997,312, 4,013,148, and 4,217,775, and British Patent No. 653,565. Most of these designs do not address the problem of vapor lock and some have other attendant limitations. A description of these designs, and a discussion of their differences and shortcomings when compared to the present invention, is included in the above-referenced U.S. Pat. No. 5,003,829, which is incorporated herein.